A black liquid solar collector cum storage system is studied. The liquid, being black, absorbs most of the radiation; also there is little reflection loss from the water surface. The performance of the system has been predicted by the finite-difference method for various operating conditions, such as intermittent flow, instantaneous flow, and constant flow. The thermal performance of the black liquid system was compared with that of a conventional and plastic bag shallow solar pond (SSP), and the SSP containing black liquid was found to be more efficient than other systems